Exclude HTML and Markdown by default

README.md

@@ -20,7 +20,7 @@ Pkg.add("ElectronDisplay")
 
 As soon as you load the package with ``using ElectronDisplay``, it will start to show plots that have the correct ``show`` methods in an electron window.
 
-`ElectronDisplay` also exports a function `electrondisplay`.  You can use `electrondisplay(x)` to show `x` explicitly in `ElectronDisplay` (e.g., when another display has higher precedence).  You can also use `electrondisplay(mime, x)` to specify a MIME to be used.
+`ElectronDisplay` also exports a function `electrondisplay`.  You can use `electrondisplay(x)` to show `x` explicitly in `ElectronDisplay` (e.g., when another display has higher precedence).  You can also use `electrondisplay(mime, x)` to specify a MIME to be used.  For example, to read a docstring of `reduce` in `ElectronDisplay`, you can use `electrondisplay(@doc reduce)`.
 
 ## Configuration
 
@@ -31,9 +31,9 @@ using ElectronDisplay
 ElectronDisplay.CONFIG.single_window = true
 ````
 
-To control objects to be handled by `ElectronDisplay`, you can set `ElectronDisplay.CONFIG.showable` (which defaults to `Base.showable`).  For example, to avoid showing help output with `?thing` to `ElectronDisplay`, you can use:
+To control objects to be handled by `ElectronDisplay`, you can set `ElectronDisplay.CONFIG.showable`.  By default, `ElectronDisplay` does not show markdown or HTML output.  To show everything in `ElectronDisplay` whenever it's supported, you can use:
 
 ````julia
 using Markdown
-ElectronDisplay.CONFIG.showable = (m, x) -> !(x isa Markdown.MD) && showable(m, x)
+ElectronDisplay.CONFIG.showable = showable
 ````

src/ElectronDisplay.jl

@@ -6,6 +6,8 @@ using Electron, Base64
 
 struct ElectronDisplayType <: Base.AbstractDisplay end
 
+electron_showable(m, x) = m ∉ ("text/html", "text/markdown") && showable(m, x)
+
 mutable struct ElectronDisplayConfig
     showable
     single_window::Bool
@@ -16,15 +18,17 @@ end
 
 Configuration for ElectronDisplay.
 
-* `showable = Base.showable`: A function with signature
-  `showable(mime::String, x::Any) :: Bool`.  This determines if object
-  `x` is displayed by `ElectronDisplay`.  Default to `Base.showable`.
+* `showable`: A callable with signature `showable(mime::String,
+  x::Any) :: Bool`.  This determines if object `x` is displayed by
+  `ElectronDisplay`.  Default is to return `false` if `mime` is
+  `"text/html"` or `"text/markdown"` and otherwise fallbacks to
+  `Base.showable(mime, x)`.
 
 * `single_window::Bool = false`: If `true`, reuse existing window for
   displaying a new content.  If `false` (default), create a new window
   for each display.
 """
-const CONFIG = ElectronDisplayConfig(showable, false)
+const CONFIG = ElectronDisplayConfig(electron_showable, false)
 
 const _window = Ref{Window}()